Pseudo mersenne reduction modulo p = 2^32+15

q = x >> 32
r = x - (q << 32)
i = 0

while (q > 0) {
 t = (q << 4) - q
 q = t >> 32
 r_i = t - (q << 32)
 if (++i % 2 == 1)
  r -= r_i
 else
  r += r_i
}

while (r >= m) {
  r = r - m
}

return r